In toll road and other applications where the passage of a vehicle is sensed, it is often required to quantify the number of axles associated with a vehicle. This is generally accomplished by means of weight sensitive treadles placed across the roadway.
Conventional treadle designs commonly employ a treadle switch unit consisting of an elastomeric envelope housing an electrical sensing unit. In one common form, the interior of the envelope is provided with spaced contact strips defined by upper and lower interior surfaces which are normally separated by air recesses running longitudinally. When the weight of a vehicle wheel is upon the treadle, it deforms the envelope and causes the contacts to engage each other (electrically) to complete a circuit. The electrical sensing means for the treadle switch may, to like effect, consist of a variable resistor which changes its electrical resistance in response to stress caused by deformation or a piezoelectric sensor which provides a voltage in response to stress. The important feature common to these electrical sensing means is that the sensor produces a readily detectible change in the current and/or voltage in response to weight applied to the unit.
These treadle switch designs all rely upon electrical current flow and are therefore particularly subject to failure due to intrusion of moisture, salts and/or other contaminants. Accordingly, the lifetime of such electrically sensing treadle designs is unduly limited. Because these treadles are typically in traffic lanes, replacement causes undesirable disruption of traffic in addition to expense.
This invention relates to a completely sealed treadle switch utilizing a fiber optic sensor of the intrinsic type. For comparison, with extrinsic fiber optic sensors, light leaves the fiber and is blocked or reflected before going back into the fiber optic system. Thus, extrinsic optical sensors have the same disadvantages as photo-electric controls in that they ar affected by dirt, contamination and mechanical vibration. By contrast, with an intrinsic fiber optic sensor, the light is processed (i.e., is detectably altered due to stress) as it passes along the fiber. This can be in the form of phase angle change or speckle pattern detection, or other attenuation.